System and method for imaging tissue

Abstract

Systems and methods of detecting a portion within tissue that has a variation of local magnetic susceptibility using an MRI device, including: transmitting a first spin-echo pulse sequence to the tissue, wherein the first spin-echo pulse sequence includes a first number of refocus pulses and a first TE value; transmitting a second spin-echo pulse sequence to the tissue, wherein the second spin-echo pulse sequence includes a second number of refocus pulses and a second TE value; obtaining a first image and a second image; determining one or more locations within the second image having a signal intensity that is different than the signal intensity of the same one or more locations within the first image; and identifying a portion of tissue that has a varied local magnetic susceptibility based on the determined one or more locations within the second image.

The invention claimed is: 
     
         1 . A method of detecting a portion within a tissue that has a variation of local magnetic susceptibility, the method comprising:
 transmitting, by a magnetic resonance imaging (MRI) device, a first spin-echo pulse sequence to the tissue, wherein the first spin-echo pulse sequence comprises a first number of refocus pulses and a first echo time (TE) value, wherein the MRI device has a nonhomogeneous magnetic field in a measurement volume such that, if conventional susceptibility-weighted-imaging were to be performed using the MRI device on the tissue located in the measurement volume, phase shifts caused by the variation of local magnetic susceptibility would need to be larger than phase shifts caused by the nonhomogeneous magnetic field in order to detect such variation, making conventional susceptibility-weighted imaging unreliable for detecting such variation of local magnetic susceptibility using such MRI device;   obtaining, by the MRI device, a first image of the tissue;   transmitting, by the MRI device, a second spin-echo pulse sequence to the tissue, wherein the second spin-echo pulse sequence comprises a second number of refocus pulses and a second TE value;   obtaining, by the MRI device, a second image of the tissue;   determining, by the MRI device, one or more locations within the second image of the tissue having a signal intensity that is different than the signal intensity of the same one or more locations within the first image of the tissue; and   identifying, by the MRI device, a portion of tissue that has a varied local magnetic susceptibility based on the determined one or more locations within the second image of the tissue,   wherein the first number of refocus pulses and the second number of refocus pulses are different, and   wherein neither the first image nor the second image are based on a signal with a non-zero phase shift caused by the nonhomogeneous magnetic field.   
     
     
         2 . The method of  claim 1 , wherein the first TE value and the second TE value are different. 
     
     
         3 . The method of  claim 1 , wherein identifying the portion of tissue further comprises identifying a location within the tissue where an effective TE of the first pulse sequence and second pulse sequence are identical. 
     
     
         4 . The method of  claim 1 , wherein identifying the portion of the tissue further comprises applying a correction matrix to the first image of the tissue and the second image of the tissue, and wherein the correction matrix is based on at least two calibration images taken using the MRI device. 
     
     
         5 . The method of  claim 1 , wherein the first number of refocus pulses is less than the second number of refocus pulses. 
     
     
         6 . The method of  claim 1 , wherein the second number of refocus pulses is less than the first number of refocus pulses. 
     
     
         7 . The method of  claim 1 , further comprising acquiring one or more calibration images from a phantom which is void of internal susceptibility gradients. 
     
     
         8 . The method of  claim 7 , further comprising:
 generating a correction matrix based on the one or more calibration images; and   applying the generated correction matrix on at least one of the first image of the tissue and the second image the second image of the tissue.   
     
     
         9 . The method of  claim 1 , wherein the tissue is a brain. 
     
     
         10 . The method of  claim 1 , wherein the MRI is a permanent magnet MRI. 
     
     
         11 . The method of  claim 1 , further comprising transmitting the identified portion of the tissue to a display. 
     
     
         12 . The method of  claim 1 , wherein the signal intensity difference between the first image of the tissue and the second image of the tissue is caused by the strength of local magnetic susceptibility gradients. 
     
     
         13 . The method of  claim 1 , further comprising:
 generating an image mask;   weighting the first image of the tissue and the second image of the tissue with relaxation time T2; and   superimposing the image mask on the weighted first image of the tissue and the second image of the tissue.   
     
     
         14 . The method of  claim 1 , wherein at least one of the first spin-echo pulse sequence and the second spin-echo pulse sequence is a 3-dimensional sequence. 
     
     
         15 . The method of  claim 1 , further comprising applying a fast spin-echo sequence to the tissue to obtain a readout of the tissue. 
     
     
         16 . The method of  claim 15 , further comprising applying multiple spin-echo pulse sequences during a predetermined time period prior to applying the fast spin-echo sequence. 
     
     
         17 . The method of  claim 1 , wherein each of the first and second images of the tissue are based on a plurality of spin echoes. 
     
     
         18 . A system for detecting a portion within a tissue that has a variation of local magnetic susceptibility, comprising:
 a magnetic resonance imaging (MRI) device having a nonhomogeneous magnetic field in a measurement volume such that, if conventional susceptibility-weighted-imaging were to be performed using the MRI device on a tissue located in the measurement volume, phase shifts caused by the variation of local magnetic susceptibility would need to be larger than phase shifts caused by the nonhomogeneous magnetic field in order to detect such variation, making conventional susceptibility-weighted imaging unreliable for detecting such variation of local magnetic susceptibility using such MRI device, the MRI device further configured to:
 transmit a first spin-echo pulse sequence to the tissue, wherein the first spin-echo pulse sequence comprises a first number of refocus pulses and a first echo time (TE) value; 
 obtain a first image of the tissue; 
 transmit a second spin-echo pulse sequence to the tissue, wherein the second spin-echo pulse sequence comprises a second number of refocus pulses and a second TE value; 
 obtain a second image of the tissue; 
 determine one or more locations within the second image of the tissue having a signal intensity that is different than the signal intensity of the same one or more locations within the first image of the tissue; and 
 identify a portion of tissue that has a varied local magnetic susceptibility based on the determined one or more locations within the second image of the tissue, 
 wherein the first number of refocus pulses and the second number of refocus pulses are different, and 
 wherein neither the first image nor the second image are based on a signal with a non-zero phase shift caused by the nonhomogeneous magnetic field. 
   
     
     
         19 . The system of  claim 18 , wherein the MRI device is further configured to identify a location within the tissue where an effective TE of the first pulse sequence and second pulse sequence are identical. 
     
     
         20 . The system of  claim 18 , wherein the MRI device is further configured to apply a correction matrix to the first image of the tissue and the second image of the tissue, and wherein the correction matrix is based on at least two calibration images taken using the MRI device. 
     
     
         21 . The system of  claim 18 , wherein the MRI device is further configured to acquire one or more calibration images from a phantom which is void of internal susceptibility gradients. 
     
     
         22 . The system of  claim 21 , wherein the MRI device is further configured to:
 generate a correction matrix based on the one or more calibration images; and   apply the generated correction matrix on at least one of the first image of the tissue and the second image the second image of the tissue.   
     
     
         23 . The system of  claim 18 , wherein the MRI is a permanent magnet MRI. 
     
     
         24 . The system of  claim 18 , wherein the MRI device is further configured to transmit the identified portion of the tissue to a display. 
     
     
         25 . The system of  claim 18 , wherein the signal intensity difference between the first image of the tissue and the second image of the tissue is caused by the strength of local magnetic susceptibility gradients. 
     
     
         26 . The system of  claim 18 , wherein the MRI device is further configured to:
 generate an image mask;   weight the first image of the tissue and the second image of the tissue with relaxation time T2; and   superimpose the image mask on the weighted first image of the tissue and the second image of the tissue.   
     
     
         27 . The system of  claim 18 , wherein at least one of the first spin-echo pulse sequence and the second spin-echo pulse sequence is a 3-dimensional sequence. 
     
     
         28 . The system of  claim 18 , wherein the MRI device is further configured to apply a fast spin-echo sequence to the tissue to obtain a readout of the tissue. 
     
     
         29 . The system of  claim 28 , wherein the MRI device is further configured to apply multiple spin-echo pulse sequences during a predetermined time period prior to applying the fast spin-echo sequence. 
     
     
         30 . The system of  claim 18 , wherein each of the first and second images of the tissue are based on a plurality of spin echoes. 
     
     
         31 . A non-transitory computer program product for detection of a portion within a tissue that has a variation of local magnetic susceptibility, the non-transitory computer program product comprising instructions which, when executed by a computer, cause the computer to:
 cause one or more components of a magnetic resonance imaging (MRI) device to transmit a first spin-echo pulse sequence to the tissue, wherein the first spin-echo pulse sequence comprises a first number of refocus pulses and a first echo time (TE) value, wherein the MRI device has a nonhomogeneous magnetic field in a measurement volume such that, if conventional susceptibility-weighted-imaging were to be performed using the MRI device on the tissue located in the measurement volume, phase shifts caused by the variation of local magnetic susceptibility would need to be larger than phase shifts caused by the nonhomogeneous magnetic field in order to detect such variation, making conventional susceptibility-weighted imaging unreliable for detecting such variation of local magnetic susceptibility using such MRI device;   obtain a first image of the tissue;   cause the one or more components of the MRI device to transmit a second spin-echo pulse sequence to the tissue, wherein the second spin-echo pulse sequence comprises a second number of refocus pulses and a second TE value;   obtain a second image of the tissue;   determine one or more locations within the second image of the tissue having a signal intensity that is different than the signal intensity of the same one or more locations within the first image of the tissue; and   identify a portion of tissue that has a varied local magnetic susceptibility based on the determined one or more locations within the second image of the tissue,   wherein the first number of refocus pulses and the second number of refocus pulses are different, and   wherein neither the first image nor the second image are based on a signal with a non-zero phase shift caused by the nonhomogeneous magnetic field.   
     
     
         32 . The non-transitory computer program product of  claim 31 , wherein each of the first and second images of the tissue are based on a plurality of spin echoes.